As the modern understanding of disease has advanced, the potential utility of cell therapy for improving the prognosis of those afflicted has resulted in increased interest in new sources of human cells useful for therapeutic purposes. One such source of human cells is postpartum tissues, and in particular, the umbilicus or umbilical cord.
Recently, attention has focused on the banking of umbilical cord blood (or simply “cord blood”) as a potential source of, for example, hematopoietic cells for use by an individual for whom cord blood has been banked at birth. Such cells would be useful for those individuals, for example, who require therapeutic radiation which may eliminate functional portions of their immune system. Rather than requiring a bone marrow donor carefully matched to avoid rejection, the individual's own banked cord blood could be used to reconstitute the lost immune cells, and restore immune function.
Still more recently, there has been interest in obtaining stem cells from cord blood, due to the wider potential therapeutic applications of such cells. Stem cells are understood in general terms as cells that 1) have the ability to self-renew for long periods through cell division from a single cell; and 2) have the ability to differentiate into specific cell types given the proper conditions. Accordingly, stem cells are potentially useful in treating a population of individuals, and not merely the person from whose cord blood the cells were initially obtained.
In particular, cord blood has been considered as a source of hematopoietic progenitor stem cells. Banked (or cryopreserved) cord blood, or stem cells isolated therefrom have been deemed useful for hematopoietic reconstitution, for example in bone marrow and related transplantations. (Boyse et al., U.S. Pat. Nos. 5,004,681 and 5,192,553).
In addition to cord blood, other sources of therapeutic cells from the human umbilicus have been explored, including cells isolated from the Wharton's Jelly, umbilical vein or artery tissue, and the umbilical matrix itself. Such cells have been largely uncharacterized, or only minimally characterized with respect to their physiological, biochemical, immunological, and genetic properties.
For example, Purchio et al. (U.S. Pat. No. 5,919,702) have isolated chondrogenic progenitor cells (or prechondrocytes) from Wharton's Jelly. They reported the isolation of cells from human umbilical cord Wharton's Jelly by removing blood and blood vessels and incubating the tissue under conditions purported to allow the prechondrocytes to proliferate. As such, the method did not distinguish the desired cells from the different cell types present in Wharton's Jelly, but rather relied on migration from the tissue or selecting growth conditions favoring prechondrocytes. The prechondrocytes were expanded mitotically after they were established. Cells at passages 2 to 4 were reported as useful to produce cartilage, if triggered by the addition of exogenous growth factors, such as BMP-13 or TGF-beta. Uses of the cells for direct injection or implantation, or use with a hydrogel or tissue matrix, were proposed. However, it was considered important that the cells not exceed about 25% confluence. The cells were not characterized with respect to their biochemical or immunological properties, or with respect to their gene expression.
Weiss et al. (U.S. Patent Application Publication US2003/0161818) proposed procedures for isolating pluripotent or lineage-committed cells from mammalian Wharton's Jelly or non-blood umbilical cord matrix sources. The cells isolated were reported to differentiate into hematopoietic, mesenchymal or neuroectodermal lines. The cell lines were not characterized with respect to their identifying properties. Limited characterization was provided with respect to cells after differentiation towards neural lines. Reference was made to Wharton's Jelly-derived bovine and porcine cells that were CD34−, CD45−.
Weiss et al. also reported investigating transplantation of porcine umbilical cord matrix cells into rat brain. (Exp. Neur. 182: 288-299, 2003). No enzymatic treatment was used in the isolation procedure. They obtained two distinct populations—spherical and flat mesenchymal cells. The cells were genetically modified to express GFP. The cells did not appear to stimulate immune rejection when implanted cross-species.
Mitchell et al. (Stem Cells 21:50-60, 2003) reported obtaining Wharton's Jelly matrix cells from porcine umbilical cords. The undifferentiated cells were reported to be positive for telomerase and a subpopulation was also reported positive for c-kit expression, i.e., telomerase+, CD117+. The cells were also reported to produce alpha-smooth muscle actin, indicative of their myofibroblast-like nature. The cells purportedly could differentiate along neural lines in the presence of growth factors. However, both the differentiated and undifferentiated cells were found to express NSE, a marker for neural stem cells. The need for clonal lines and characterization in terms of proliferative capacity, karyotype analysis, and expression of HLA antigens was recognized.
Romanov et al. (Stem Cells 21(1):105-110, 2003) reported a procedure to isolate mesenchymal stem cell (MSC)-like cells from human umbilical cord vein. Their procedure involved treatment of the excised vein tissue with collagenase and required that the enzymatic digestion be short (15 minutes) to obtain the cell population of interest (a subendothelial layer of the vein). In particular, the procedure avoided inclusion of smooth muscle cells (SMCs) and fibroblasts by leaving the deeper layers of the tissue intact, purportedly removing only the outer layers. The resulting “nearly homogenous” cell population was reported to contain approximately 0.5-1% endothelial cells that they also sought to avoid. The cells were reported to be predominantly CD34− and to produce alpha-smooth muscle actin.
Because of the diversity of cell populations that are found in umbilical cord matrix, there is a need in the art for methods of isolating defined non-blood cells and populations thereof derived from mammalian umbilical cord; as well as a need for cell lines derived from mammalian umbilicus that are characterized with respect to their biochemistry (e.g. secretion of growth factors), immunology (cell surface markers and potential to stimulate immune responses) and expression of various genes. This need is particularly compelling for cells derived from human umbilicus.
With regard to media for culturing cells most contain at least some fetal bovine or calf serum. Generally, commercially available media formulations utilize serum supplements of about 10-20% (v/v). The serum component is often integral to the survival and expansion of cell populations. A myriad of proteins are found in bovine serum, including for example PDGF and FGFs, known growth factors that can have significant influence on cell growth and differentiation on cell populations, including populations of stem and progenitor cells.
In spite of the advantages of including bovine serum (or serum of other species), there are, however, a number of disadvantages to supplementing medium with animal serum instead of using chemically-defined or serum-free media when culturing therapeutic cells or producing biologics. Cell and tissue homeostasis occurs under environmental conditions that lack blood-derived serum. Thus, long-term cell exposure to serum and blood-related products simulates a tissue injury paradigm. Further, lot-to-lot variation in composition of serum, including in the stimulatory proteins and any inhibitory substances requires time consuming and expensive pre-testing to ensure each batch meets the standards for therapeutic product development or production. Finally, increased concerns about the transmission of diseases such as bovine spongiform encephalopathy (BSE) from the use of animal-related products may ultimately retard or preclude FDA approval of cell-based therapies developed with foreign serum.